I am a borderline terrible player not going to lie. That having been said what I have managed so far is beat it with Metropolis solo, Darkness solo, Nature solo. I have beat all four maps. I have watched the credits start to finish. My wizard is level 69. My usual run if I am playing is a 500 rainbow run or Darkness Nature. I usually only go 200 deep if I plan to do a specific quest.
My favourite strategy so far is pretty much brute force Prince Lycaon, or Shadow Mane as a hero. With the plan of marrying a Hitman (Ring, Impatience's Wrath and Excalibur) and my primary Shadow (Ring, A Strong Force, Mess Reaver, Blade of Darkness) together for a AoE/Single target combo. Abyss King (Helm of hades), and Knusperhexe work together to give my primary Shadow a good field of fire.
I have recently switched over to Lord Roderic as my hero, however it makes a Hitman a very chancy piece due to the slow attack speed to start so I am trying some different things.
As for the factions.
Nature I honestly don't know how to use very well, my rainbow build sees me place 2-3 Huli's for them making enemy units slip. A couple of Beavers for the Stun. The Herb Witch ideally buffing my Hitman and Shadow.
Darkness I understand some but Scarecrow, Solara, Gib, Acolyte of Greed, The Ripper all seem to not make a lot of sense as a new player.
Metropolis I use Pocket Thief as early filler if I just need a spot occupied but its never key other than being where I drop my Mr. Iron ideally. Electric Chair and Elvis Imitator I haven't got a hang on. Irish Pub seems really lackluster but I build 1 or 2 on some maps and usually remember to trigger the ability for Challenges. Money Bin I have a tendency to buy one early and then sell it once my last purple tower goes down. Scarface used to be my favourite place to roll out the full withered set, but now I usually just put on it on my Mr Iron once I hit bonus round. Muli the evil twin has never managed anything useful for me. Biofeld Laser Satellite has worked just once for me when I had something silly like 8 money bins on a map and used it to replace my Hitman for the bonus rounds. Black Widow and Stonecutter's Temple seem straightforward even if Stonecutter's confuses me on whether or not it needs to be 99 level or the tower in question needs to be.
At this point what should I focus on as a player? Is crafting legendary items the way to go or is it better to Black Market once first? Or is there something else I am missing. I never played the original game that this drew inspiration from.
Hitman is one of the most fun towers to use, however, they are also one of the worst long-term prospects. Their absolutely wild damage swings (Can go as low as 1 damage, don't forget) make them extremely inconsistent, their slow rate-of-fire makes them almost impossible to get up to max speed with, and they lack any scaling abilities.
For straight Metropolis, Mr. Iron loses access to the best item for him: The Blood Demon Blade. They also lose their best carry in Muli, as he can't attack without a nature deck to support him. Blackwidow unfortunately lacks long-term punch, and should be treated like a support tower. Stonecutter's Temple can be a nice "backup carry" if needed. Also, his bonus applies from his level, not the other towers. So a Level 1 Hitman will still receive the bonus of a Level 99 Stonecutter's Temple, but not vice versa. The Blofeld can be fun, but as you noticed, it will almost begin to perform worse as the bonus round drags on.
Nature is a bit more of an interesting solo deck. Of its three Legendary towers, two are supports, leaving Mr. Cuddles as the sole carry (Which he cannot do well on his own). A wolfpack (Making basically every tower Wolves) can easily get to the bonus round, but will rapidly run out of steam (Unless you replace all those high-level Wolves with other towers). Holgar is the closest thing you'll get to a Nature carry, and even then - he doesn't have a hope in hell later on in the bonus rounds. Nature, even moreso than Metropolis, should be thought of as a support deck only.
Which brings us to... Dedicated Darkness! Unlike the other two, this deck is absolutely teeming in awesome carry towers. Solora, Scarecrow, Shadow, Ripper and Dark Forge can all be used as dedicated carries, and they also have the two most offensive-orientated support towers in the game (Knuperhexe and Abyss King). Literally any of them can be used, though trying to run Ripper can be extremely inconsistent (You'll either dominate the bonus round, or fail to clear the initial 500).
In terms of focus, I would STRONGLY recommend you focus on what you want, and then craft those exact things. The Black Market is awesome for a couple things, but 60% of it is pointless stuff.